1. Field of the Invention
The present invention relates to the field of bore hole surveying
2. Prior Art.
In the drilling for gas and oil it is generally desired to know the trajectory of the drill string for various reasons. Ideally it would be desirable to be able to monitor and plot the path of the drilling as drilling progresses. However, the extreme environment and other factors generally prevent this, with the result that normally the drill string is pulled periodically and appropriate sensors put down the hole to take the required readings from which the drill string trajectory can be determined. See for instance U.S. Pat. Nos. 2,635,349; 3,384,750; 3,753,296; 3,896,412; and 4,071,959. While the nature of the instrumentation lowered into the hole and the concept of operation thereof varies widely, it is common to measure as one reading the length of cable let out as one of the primary data inputs while other readings are simultaneously being taken.
Such a system is also shown in U.S. Pat. No. 4,238,889. That patent discloses a system having a two axis gyro and a two axis accelerometer mounted on the inner frame of a two axis platform. While the system of that patent utilizes a belt drive to couple the inner frame with its associated angular position sensor and torquer so as to space the angular position sensor and torquer at different axial positions along the bore hole, the two axis gyro together with the two axis accelerometer are all mounted coaxially, with the result that the overall assembly in accordance with that patent is still considerably larger than that of the present invention.